Historically a combination of iron and nickel or cobalt has been used to produce low coefficient of thermal expansion (CTE) alloys. Invar 36 (36 Ni, balance Fe), Invar 42 (42 Ni, balance Fe) and Kovar (29 Ni, 17 Co, 0.2 Mn balance Fe) are common low coefficient of thermal expansion alloys. Welding of iron containing low CTE alloys requires a weld with adequate ductility and strength. In addition, for fabrication of welded low CTE alloys it is desired to use a filler metal with a CTE closely matched to the base metal to limit thermal deflections and strain.
Without further modification, low CTE base metals may be easily drawn into filler metal wire. However, base metal filler metal is highly subject to cracking resulting from impurities and a relatively large solidification range of about 80.degree. Celsius. Witherell, in U.S. Pat. No. 3,184,577 ('577), disclosed a filler metal specifically designed for welding Invar. A combination of titanium, carbon and manganese was used in the '577 patent to control cracking during welding operations. Commercially, filler metal of the '577 patent has been used for welding Invar 36 with undesirable limitations. Filler metal disclosed in the '577 patent cracks extensively when welding relatively thick sections of Invar. In addition, filler metal of the '577 patent has been limited to relatively slow gas tungsten arc welding (GTAW), because cracking is excessive when welding with gas metal arc welding (GMAW). GMAW is significantly quicker than GTAW and is generally preferred for high volume fabrication. Furthermore, multiple pass welding with Witherell's filler metal has caused excessive crack growth problems.
In addition, submerged arc welding (SAW) may be used to further increase welding rates. Submerged arc welding operability of a filler metal is another highly advantageous property. Furthermore, flux coated electrodes are useful for out of position welding where it is difficult to weld by GTAW, GMAW or SAW.
It is an object of this invention to produce a welding material having a coefficient of thermal expansion that matches the base metal with sufficient strength and ductility.
It is a further object of this invention to produce a filler metal capable of welding relatively thick sections of Invar without cracking.
It is a further object of this invention to provide a filler metal that may be gas metal arc welded without cracking.
It is a further object to produce a welding material that may be used in multiple pass type welding operations without cracking.
It is a further object of the invention to produce a weld wire that may be submerged arc welded.